This invention relates generally to the field of fiber-optic transmission systems and, more specifically, to managing dispersion within an optical add/drop multiplexer (OADM).
Within an optical communications system such as a wavelength division multiplexed (WDM) or dense WDM (DWDM) system, optical add/drop multiplexers (OADMs) are used to insert (add), remove (drop) or pass through individual optical channels or groups of channels at one or more intermediate locations along an optical fiber path. In long distance WDM systems, transmitted optical signals are subjected to non-linear effects such as self-phase modulation or cross-phase modulation, which effects degrade the transmission performance and quality of the optical signals. Other effects such as pulse broadening due to chromatic dispersion also degrade the transmitted optical signal.
To combat the non-linear and chromatic dispersion effects upon an optical signal due to various characteristics of an optical transmission medium, it is known to impart a complimentary pre-distortion to the optical signal prior to transmission. Post-transmission optical processing may be employed to further mitigate the impact of non-linear and dispersion effects. At an OADM, those optical channels to be passed through bypass pre- and post-transmission dispersion compensation using, for example, an optical blocking filter that passes only the through-channels directly from the optical demultiplexer output to the optical multiplexer input of the OADM.
While the above-described OADM performs its intended function, the cost for implementing such an OADM is relatively high.
The invention comprises an apparatus and method for implementing an optical add/drop multiplexer (OADM) function in a relatively cost effective manner. Specifically, an OADM according to an embodiment of the present invention imparts a first dispersion compensation to each of the optical channels within a WDM or DWDM signal, extracts those optical channels to be dropped, combines pass-through optical channels and those optical channels to be added and performs a second dispersion compensation on the resulting combined WDM or DWDM signal. The second and first dispersion compensation functions are constrained as follows:
The second compensation function is selected to be appropriate for channels to be added (i.e., equal to the xe2x80x9cpre-compensationxe2x80x9d normally used within the specific system), and the first dispersion compensation is selected such that the sum of the first and second dispersion compensations is substantially zero. In various embodiments, the second dispersion compensation is negative, while the first dispersion compensation is positive. Advantageously, a positive dispersion may be realized using inexpensive standard single mode fibers (SSMFs).